Chad Hanna was enjoying a quiet Christmas night with family in rural western Pennsylvania when he got the text message. He sprang for his phone, surprising his in-laws. Then he grabbed his laptop and flew up the stairs to an empty bedroom.

The cosmos had quietly gifted scientists with a second gravitational wave, dubbed GW151226, from two black holes that collided 1.4 billion light-years away. The signal from those black holes — one 14 solar masses and the other eight — showed the final d